The invention relates to a process for protecting an information item transmitted from a security element such as, for example, a user-card of a conditional-access system, to a decoder.
The invention applies more particularly to conditional-access systems for which the descrambling operation is performed in the security element which is then, for example, a PCMCIA type card complying with the interface standard known by those skilled in the art as "CENELEC/DVB-Common Interface" or a chip card complying with the American NRSS standard (standing for "National Renewable Security System").
The invention applies to any type of conditional-access system, whether this system be of the on-line type or the standalone type.
In an on-line conditional-access system, the scrambled information item is an information item consisting of a signal distributed simultaneously to various users.
In a standalone conditional-access system, the scrambled information item is contained on standalone information media such as, for example, compact discs or digital video discs.
The information item making up the various programmes originating from the issuing source, such as, for example, a service provider, is transmitted to the security element. The security element descrambles the programme selected by the user (provided that the user's entitlements are present in the security element) and sends this programme, as well as the other programmes which have remained unchanged, to the decoder.
Such a process has the drawback that the programme selected by the user is transmitted unenciphered.
Such a transmission can readily be exploited by a pirate who can use it to distribute the pirated programme illegally.
FIG. 1 represents the schematic of a security element/decoder assembly according to the prior art.
The system of FIG. 1 comprises an information source I, a decoder 6 and a security element 1.
The decoder comprises a demodulation device 7 and a demultiplexing and decoding device 8.
The security element 1 contains a filtering device 2, a descrambling device 3, an access control device 4 and a user entitlement storage device 5.
The information item I issued by the issuing source contains one or more multiplexed programmes, for example, according to the MPEG-2 transport standard (standing for "Moving Picture Expert Group").
As is known to those skilled in the art, the programmes output by the issuing source are scrambled programmes. The information item I contains, in messages which will hereafter be denoted ECM, the encrypted control words allowing, after decryption, the descrambling of the scrambled programmes.
After the decoder receives the information item I, the latter is demodulated by the device 7 and then transmitted in full to the security element 1. The latter filters, with the aid of the device 2, the ECMs (denoted ECMA in FIG. 1) corresponding to the programme selected by the user and transmits them to the device 4 for processing. The non-filtered part of the information item is transmitted without modification to the descrambler 3. The device 4 carries out the conventional functions for processing the ECMs, and, in particular, decrypts the control words CWi which they contain, provided that the entitlements D necessary for descrambling the selected programme and output by the device 5 are applied to the device 4.
The control words CWi are subsequently transmitted to the descrambling device 3 which uses them to descramble the programme selected by the user. The information item output by the descrambler 3 is transmitted to the demultiplexing and decoding device 8 so as to generate the usable, i.e., for example, displayable in the case of a film, information item ECG1.